It is known to use hydrophobic or hydrophobicized particles or silicone resin particles in cosmetic formulations, e.g., as a matting agent, as absorbers for skin sebums or for producing a silky skin feel. Moreover, corresponding particles are often used in make-up formulations in order to improve the holding power of the make-up on the skin. Silicone-rubber powders or powders of silicone elastomers are often used for this purpose.
Several methods are known for producing corresponding particles. In principle, irregularly shaped silicone elastomer particles can be obtained by grinding processes of the respective bulk elastomers, but spheroidal or spherical particles generally offer application-related advantages, particularly if pleasant tactile properties of particle-additivated materials and formulations are desired. Usually, such particles are produced by crosslinking reactions within starting material droplets or growing on/application of a polymer to a particle core. Crosslinking reactions can be, e.g., hydrosilylation reactions, condensation reactions, dehydrogenative coupling reactions or free-radical polymerization.
Silicone particles of hydrosilylations are described, for example in U.S. Pat. No. 4,761,454, JP 2003301047 and EP 1 074 575, hydrolysis and condensation reactions for producing silicone particles can be found in EP 1 130 046, WO 2006/016968, JP 2003002973, U.S. Pat. No. 6,753,399, EP 0 765 896 and EP 0 744 432, while U.S. Patent Publication No. 2004/0156808 describes a dehydrogenative coupling reaction for this purpose. Finally, DE 10 2004 053 314 describes copolymers of at least two different macromonomers obtainable by free-radical polymerizations.
The production of free-radically crosslinked silicone acrylate particles was reported for the first time in 2005 in “Polydimethyl siloxane latexes and copolymers by polymerization and polyaddition in miniemulsion” (Katharina Landfester, Ute Pawelzik, Markus Antonietti, Polymer, 46 (2005), 9892-9898). In the aforementioned publication, the miniemulsion polymerization of silicone acrylates to nanoscale particles using a conventional emulsifier and a customary molecular radical initiator, such as, for example, AIBN, is described in detail. However, the described process does not produce microscale particles which have the desired application-related properties, i.e., good skin feel desired for personal care applications cannot be obtained with such particles.
Solid-body-stabilized, aqueous emulsions were described in 1907 by S.U. Pickering (“Emulsions”, Spencer Umfreville Pickering, Journal of the Chemical Society, Transactions (1907), 91, 2001 2021) and are considered to be particularly stable to coalescence. Thus, DE 10 2004 014 704 describes the preparation of emulsions which are stabilized with pyrogenically produced particles. A good overview of the properties of such stabilizing solids particles can be found in “Particles as surfactants—similarities and differences” by Bernard P. Binks (Current opinion in colloid & interface science, 7 (2002), 21-41). The prior art also includes so-called “Janus particles”, amphiphilic particles with a hemispherically modified surface, as described e.g. in FR 2 808 704. Of particularly good suitability for emulsion stabilization are nanoscale, predominantly inorganic particles, e.g., silica particles which are commercially available as “LUDOX®” in the form of aqueous sols or dispersions from Grace Davison. U.S. Pat. No. 3,615,972 (1967) describes for the first time the use of LUDOX® particles for the emulsion stabilization of methyl methacrylate with subsequent polymerization. The mechanism of the stabilizing effect discussed in the literature is the agglomeration of the particles and the accumulation of the agglomerates at the water/oil interface (“The mechanism of emulsion stabilization by small silica (LUDOX®) particles”, Helen Hassander, Beatrice Johansson, Bertil Törnell, Colloids and Surfaces, 40, (1989), 93-105).
The suspension polymerization of Pickering emulsions of insoluble or poorly soluble starting materials is usually started by means of a radical initiator dissolved in the oil phase. The use of water-soluble radical initiators with styrene as the sole monomer leads to incomplete conversion and coagulation (“Pickering stabilized miniemulsion polymerization: Preparation of clay armoured latexes”, Séverine Cauvin, Patrick J. Colver, and Stefan A. F. Bon, Macromolecules 2005, 38, 7887-7889).
DE 102007058713 describes a process for the preparation of microscale silicone (meth)acrylate particles by suspension polymerization using a solid-body emulsifier and a water-soluble redox radical initiator.
JP 2006-070378 describes a process for preparing dispersions comprising silicone (meth)acrylate particles. The preparation takes place using an emulsifier and a water-soluble initiator. JP 2000-063462 describes the preparation of hydrophilic siloxane latex emulsions.
One disadvantage of this process is that high concentrations of a water-soluble radical initiator are necessary in order to ensure adequate crosslinking of the particles. Moreover, these high radical initiator concentrations require the use of large amounts of a potassium phosphate buffer in order to keep the pH of the system constant during the polymerization. As a result, this process results in an ecologically undesired high salt burden of the wastewater resulting from the process. Moreover, complex, cost-intensive washing cycles are required in order to remove residues of radical initiator and buffer from the product after the polymerization.
A further disadvantage of the silicone (meth)acrylate particles produced in this way is that the polymerized particles have a relatively high content of (meth)acrylate groups that have not fully reacted, but are bonded covalently to the polymer. These often lead to an unpleasant odor being perceptible in the presence of ester-cleaving or transesterifying substances, such as for example, water or alcohols, and this odor makes the particles unusable for various applications, especially those which come into contact with the consumer.
Moreover, the particles prepared in this way have the disadvantage that they mostly have a yellowish-brownish discoloration as a result of the action of the redox radical initiator, which is undesired particularly for applications in the cosmetic sector. As a result, the preparation of the particles often requires an additional bleaching step, for example with H2O2, which is unfavourable both from an ecological and economical point of view.
A further disadvantage of the particles prepared according to DE 102007058713 is that they have irregular, nonspherical structures, which has an adverse effect on the skin feel of the particles.
Besides the preparation, the use of a very wide variety of silicone-based particles in cosmetic compositions is also described in a number of publications.
Thus, for example, EP 0834305 describes gel-like cosmetic skin-treatment compositions which comprise spherical powders of organopolysiloxanes with an average particle size from 1 to 15 μm. The particles added are in particular hydrophobicized and/or silicone-treated inorganic powders.
EP 0765656 describes cosmetic water-in-oil emulsions as cosmetic compositions which comprise powders of spherical, elastomeric organopolysiloxane particles. Besides the elastomeric (mouldable) particles, these compositions have hydrophobicized silica particles.
FR 2682534 describes skin cosmetics which have two different particle fractions, where the one fraction consists of non-mouldable particles, preferably glass beads, and the other consists of mouldable, i.e., elastic, particles.
The common feature of the cited documents is that a pleasant (soft), powdery skin feel is to be achieved through the use of the elastomer particles.
The use of non-elastomeric silicone acrylate and methacrylate particles in cosmetic applications was reported for the first time in DE 102007058713 and in WO 2012/100884 A1. The particles described in WO 2012/100884 A1 have a non-optimal skin feel. Moreover, these silicone (meth)acrylate particles in the cosmetic compositions have the above-described disadvantages attributable to their preparation process based on a water-soluble radical initiator; in particular, the particles have dimples (indentations), which has an adverse effect on their skin feel.